Starch is of a mixture of amylose (15-30% w/w) and amylopectin (70-85% w/w). Amylose consists of linear chains of α-1,4-linked glucose units having a molecular weight (MW) from about 60,000 to about 800,000. Amylopectin is a branched polymer containing α-1,6 branch points every 24-30 glucose units. Its MW may be as high as 100 million.
Sugars from starch, in the form of concentrated dextrose syrups, are currently produced by an enzyme catalyzed process involving: (1) liquefaction (or thinning) of solid starch with an α-amylase into dextrins having an average degree of polymerization of about 7-10; and (2) saccharification of the resulting liquefied starch (i.e. starch hydrolysate) with amyloglucosidase (also called glucoamylase or GA). The resulting syrup has a high glucose content. Much of the glucose syrup, which is commercially produced, is subsequently enzymatically isomerized to a dextrose/fructose mixture known as isosyrup.
Alpha (α)-Amylases (α-1,4-glucan-4-glucanohydrolases, E.C. 3.2.1.1) are a group of enzymes that hydrolyze starch, glycogen, and related polysaccharides by cleaving internal α-1,4-glucosidic bonds at random. This enzyme class has a number of important commercial applications in, for example, in the initial stages (liquefaction) of starch processing, in textile desizing, in deinking of recycled paper, in starch modification in the paper and pulp industry, in wet corn milling, in alcohol production, in sweetener (e.g., sugar) manufacture, in the beverage industry, in brewing, in oilfields, in animal feed, and as cleaning agents in detergent matrices. For example, such enzymes can be used to remove starchy stains during dishwashing and laundry washing.
α-amylases are isolated from a wide variety of bacterial, fungal, plant and animal sources. Industrially, many important α-amylases are those isolated from Bacilli. One characterized α-amylase is that of an alkaliphilic Bacillus sp. strain TS-23 which produces at least five kinds of enzymes exhibiting starch hydrolyzing activity. (Lin et al., 1998, Production and properties of a raw-starch-degrading amylase from the thermophilic and alkaliphilic Bacillus sp. TS-23, Biotechnol. Appl. Biochem. 28:61-68). The α-amylase of Bacillus sp. no. TS-23 has a pH optimum of 9 although it is stable over a broad pH range (i.e.; pH 4.7 to 10.8). Its temperature optimum is 45° C., although the enzyme has activity at lower temperatures, e.g., 15-20° C.
There remains a need for variant α-amylases that posses altered biochemical characteristics and offer improved performance in the industrial applications.